Helium-oxygen mixture at normobaric and thermoneutral conditions is regarded devoid of pharmacological activity. Physiological effects thus far reported have largely been attributable to its high thermal conductive characteristics, to its low specific density, and to its facilitation of diffusion of other gas species. However, several specific effects of helium gas have been reported. The specific effects of helium on the cardiovascular system has come to limited attention only in recent years, notably the anti-arrhythmic effect, bradycardiac effect, and circulatory catecholamine suppressive effect. The mechanisms by which helium gas exerts its effect is unknown. This project is designed to study systematically the cardiovascular actions and possible mechanisms of actions of respiring helium. The roll of the autonomic nervous system in the induction of cardiac arrhythmias is well known and the possibility exists that helium may be exerting an anti-arrhythmic effect via an action on the sympathetic nervous system. Possible involvement of autonomic nervous system will be investigated. In sequence, we plan to establish the effect of respiring helium on the cardiovascular system, on the isolated perfused heart, on the isolated atria, on specific sympathetic effector system, and then on the release and metabolic processes of catecholamines from the heart and adrenal gland. BIBLIOGRAPHIC REFERENCES: Lin, Y. C. and S.M.C. Lum. Effect of helium on the performance of the isolated perfused rat heart. Federation Proc. 34: 457, 1975. Lin, Y. C. and Kobayashi, R. H. Cardiovascular functions of the unanesthetized small Indian mongoose, Herpestes auropunctatus. Accepted for publication in Comp. Biochem. Physiol., 1975.